It is known to apply extension pieces to dental implants which are implanted in a patient's jaw bone, said extension pieces serving as a support structure for tooth attachments. Such extension pieces are often also called abutments or secondary parts.
The extension pieces are also used for fixing tooth prostheses with several teeth. A tooth prosthesis is typically secured on several such extension pieces held on implants. In this connection, it is important that the axial direction of the extension pieces is parallel, even when the fitted implants are not precisely parallel, so that the prosthesis can be applied in a defined direction of application. It is already known to machine extension pieces so that a defined direction of application can be achieved (see, for example, Technoinfo “Die Anwendung des Fräszylinders für das Octa-System” [Use of the milling cylinder for the Octa system] by Straumann).
It is also known to provide the extension piece with what is called a ball anchor. The extension piece which can be screwed into the implant is in this case provided on its top face with a ball-like projection on which a prosthesis can be held by means of corresponding holding elements in the manner of a press-button (see, for example, Technoinfo, “Die Anwendung des retentiven Kugelankers” [Use of the retentive ball anchor] by Straumann or “Das Bone Graft System” [The bone Graft System] by Straumann).
It is also generally known to transfer the situation of implants and extension pieces in a patient's mouth to a working model. The dental technician then produces the tooth prosthesis using the position of the implants and of the extension pieces in the working model. To transfer the situation to the working model, impression-taking elements are used which transfer the axial and radial position of implant and extension piece into an impression compound. Impression caps are known which are screwed to the implant. However, impression caps are also known which can be snapped or clamped onto an implant shoulder (see, for example, EP 879 024) or onto an extension piece (see, for example, DE 44 15670). From WO 02/087461, an implant is also known in which the implant is secured in a storage container by means of an impression cap.
The impression cap is used while an impression is being taken of the situation in the patient's mouth, and it remains in the impression compound when the impression compound is removed from the implant.
However, all of these known extension pieces and transfer aids are associated with certain disadvantages if the extension piece serves as a basis for a retention element for a tooth prosthesis. Particularly in the case of implants which are not fitted exactly parallel to one another, it is difficult to machine and fit a plurality of extension pieces in such a way that the direction of application of a prosthesis is parallel for all the fitted extension pieces. The use of known impression-taking systems is unsatisfactory for this purpose. Known impression-taking systems do not permit an impression of the course of the internal thread on the implant, because the thread start is not in a predefined relationship to reference surfaces, for example an internal octagon, provided on an implant. It is therefore not possible to take an impression of the thread course by referencing of such surfaces.
A determination of the position of the implant in the circumferential direction is not possible when using solid secondary parts, because this secondary part/extension piece covers the reference surfaces provided on the internal contour of the implant for definition of the circumferential direction.
The secondary part disclosed in EP 879 924 serves in particular for supporting a single tooth replacement. The secondary part is not suitable as a basis for a retention element. Taking an impression on a non-rotationally symmetrical reference surface of the extension piece, as shown in EP 879 024, is likewise associated with disadvantages in terms of the use of the extension piece as a basis for a retention element. On the one hand, the exact position of the extension piece in the axial direction and in the circumferential direction is not precisely defined because of the mutual bearing of the extension piece and the internal contour of the implant in a relatively steep cone surface.
On the other hand, the relationship between the thread start of the extension piece and the reference surface on the extension piece is not always exactly defined in the case of extension pieces which serve as a basis for a retention element, and which are typically made of precious metal. The use of a manipulation implant with an element identical to the extension piece and formed integrally with the manipulation implant (as shown in EP 879 024) is therefore also difficult.